1. Field of the Invention
This invention relates to the field of utilizing flue gas recirculation on solid fuel fired boilers and furnaces.
2. Prior Art
Flue gas recirculation (FGR) in boilers and furnaces is a technique generally known in the prior art. Flue gas is commonly introduced into a combustion chamber within a boiler above the burning fuel bed. This continual recycling of the flue gas results in a further burning of smoke and other particulate matter contained therein. Additionally, the formation of various nitric and nitrous oxides and carbon monoxide gases in the flue gas is reduced thereby minimizing the releasing of these undesirable gases into the atmosphere.
For example, U.S. Pat. No. 4,089,278 issued to Brandt is typical of most FGR apparatus. Brandt teaches the removal of flue gases from a stack and directing these flue gases through a conduit and then into the combustion chamber above the burning fuel. This technique is also utilized in liquid or gas fired systems, see for example Carson et al U.S. Pat. No. 3,854,455. It should be apparent that systems which utilize a fluid or gaseous fuel do not possess all of the same set of problems associated with coal fired boilers and furnaces. For example, fluid or gaseous fuels do not have the problems of clinker formation and do not involve fuel bed structures.
A different approach to the use of FGR is disclosed in U.S. Pat. No. 2,561,717 issued to Vicard. Vicard teaches removing flue gas from the stack of a furnace or boiler and dividing the gas into two different portions. The first portion, which is entirely flue gas, is channeled to the fuel bed prior to the area at which combustion is taking place. Vicard uses this first portion of flue gas to preheat and dry the coal prior to combustion. To the extent that the fuel bed is charged with nearly 100% flue gas (which is non-combustible) the fuel bed will not burn. Vicard uses this fact to prevent combustion from occuring where it is not desired. This is one object of FGR as used in Vicard. The second portion of flue gas is combined with air and water vapor produced by an atomizer in a chamber under the combustion portion of the fuel bed. The object of this second portion of FGR in Vicard is to use the flue gas-water vapor mixture to reduce slag formation by introducing steam with the combustion air. That is, the flue gas is used to heat added water droplets to form steam which then passes through the fuel bed.
Other prior art references which applicants are aware of are U.S. Pat. Nos. 3,781,162, 3,277,945, 3,905,745, and 3,892,191. None of the prior art references discloses the overall use of recirculated flue gas for coal fired boilers where the flue gas is introduced into the primary undergrate combustion air for reducing noxious pollutants (e.g. NO.sub.x and carbon monoxide), reducing fuel bed clinkering and reducing smoke emissions. The main advantages of the present invention will be discussed in greater detail hereinafter.